Method of heat-treating steel



Patented June 7, 1949 UNITED STATES ENT OFFICE and Frederick C. Youn ors to Ford Motor Com corporation of Delawa g, Detroit, Mich., assignpany, Dearborn, Mich, a re No Drawing. Application February 5, 1941, Serial No. 377,540

face of the steel which it was found produced a hard outer surface. However, in order to obtain the desired hard outer surface it was found to be necessary to use various and relatively expensive alloy steels.

Exceptionally fine results have been obtained by us in a recently developed process for nonalloy steels by the use of a mixture of ammonia gas and carbon monoxide or a combination of ammonia gas and generator gas in which the artiticles to be nitrided are heated in the presence of 5.

NH3 and CO at a temperature range of from 1200 F. to 1500 F. In the early stages of the development of the present process NHs and CO gas was used exclusively in treating various articles including cylinder liners but the results ob- :5

tained were not sufficiently uniform as a conside able number of the articles that were returned from service showed considerable variation in ability to withstand wear, tion in the wearing properties of the product investigations were begun to determine the cause of such variations and as a corollary of such investigation the discovery of the process of the invention disclosed and claimed herein, resulted. Studies were first instituted to investigate the physical characteristics of the steel structure by means of photomicrographs of cross sections of various samples of steel structures treated. The earlier part of the investigations were directed to improving the technique of etching samples in order to brin out the structures more com pletely, and after much study, it was determined that when non-alloy steels are subjected t a nitriding process, the hardened layer formed on the outside of the steel may be made up of two layers of distinct composition. The outside layer was generally made up of a characteristic nitrided structure, the depth of which was not entirely uniform but varied over the samples tested and this layer was next followed by an austenitic structure under the aforesaid outer nitride layer. The next layer was of characteristic martensitic structure. Although samples having the nitride layer at the outside were generally of reasonably satisfactory properties, it was found that certain Because of this varia- 2 Claims. (Cl. 14816.5)

samples gave exceptional wearing qualities and service and this was particularly noted on samples where the outer surface had been honed, therefore it was determined that exposure of the inner austenitic structure was probably responsible for the extremely good Wearing qualities of these samples. It was also found that certain other samples exhibited an austenitic structure at the outside of the steel without the formation of the nitride layer, or at least, this nitride layer was not present at the final condition of the treated steel, These samples had exceptional wearing qualities and were, of course, far superior to the other samples tested. At this stage in the development, however, these samples could not be produced with uniformity and although the austenitic structure at the surface was admittedly the reason for the superior properties, the conditions of heat treatment which would consistently produce such a product were not known. It is a known metallurgical fact that a steel with an austenitic structure has marked advantages in wearing quality over other steels, but, heretofore it has only been possible to produce such a structure by resorting to severe quenching methods on steels of ordinary composition, or by the use of large amounts of relatively expensive alloy ing elements in the steel. One of the most sat isfactory steels capable of producing an austenitic structure is the so-called Hadfield high manga nese steel, which is of the latter special composition type containing 10 to 15 percent manganese with a carbon range of 1 to 1.5 percent. With both composition and heat treatment requiring special consideration, the mass production of cer tain steel parts with an austenitic structure from Hadfield steel is not commercially practicable because of the prohibitive cost. Therefore the possibility of consistently producing a new and superior product with an austenite surface from so-cailed common steels by methods not materially more complicated than ordinary case hardening was considered worth careful investigation.

Continuing with the experiments, it was, of course, considered that if some samples were obtained with the very superior, austenitic long wearing surface, there must be a definite reason' for the formation thereof and if the conditions under which such structures were produced could be properly controlled, it would be possible to obtain this structure at the surface of the steel consistently on all articles treated. It was the discovery of this method of treatment capable of consistently and uniformly producing this austenitic surface structure that is the gist of the present invention. In essentials the desired results were found to be capable of being accomplished by a control of temperature wthin a definite range as well as by controlling the ratio between ammonia gas and generator gas used in the process within a specified range, and with further consideration for the rate of cooling of the treated metal. A considerable period of experiment was required to determine the rangeunder which uniformly satisfactory results would be produced.

While it was known that severe but carefully controlled quenching will sometimes produce an austenitic structure in ordinary-steels, it was real-. ized that other methods capable of more ac-- curate control would of necessity have to be,

found particularly for use in the high speed, mass production automotive industry. The necessity for the severe quenching of ordinary steels is due to the fact that the transformation through the austenitic stage is extremely rapid in such steels, therefore it was desirable to avoid such quenching, not-only because of the difficulties of accurate control, but also because of the danger of distortion, warping and fracture of theparts produced.

The particular range of control which was found necessary in order to produce the desired results was determined by a consideration of a lengthy series of tests, the treated steel from each test being studied by the use of microphotographs of etched sections through the steel produced.

In certain tests the furnace temperature was held at 1200 F., while the NI-Iz was fed at the rate of 1000 cu. ft. per hour and generator gas at the rateof 1300 cu. ft. per hour. From microphotographs of samples-produced. from these tests it appeared that the steel structure consisted of an. outer nitride layer with an austenitic structure beneath it followed by a martensitic layer which was the normal result of thenitriding process formerly used. In the next series-of tests the furnace was also held at a constantv temperature of 1200 F. but the NH3- was fed'at. the rate, of 1500 cu. ft. per hour and the generator gas at the rate of 1300 cu. ft. per hour, thus changing the ratio of NH3 to generator gas. In another series of tests furnace temperature and generator gas conditions were the same but the NH; was increased. to 2000 cu. ft'. per-hour. The result was still a definite outer nitride layer-but of somewhat less thickness followed by the aus tenitic inner layer.

Inlater series of. tests'the furnace temperature was held at 1300 F. with NH: fed at 1000 cu. ft. per hour and generator gas at 1300 cu. ft. per hour. This still resulted in an outer nitride layer asin the previous tests.

However, on the next series of tests the furnace temperature was held at 1300 the NH; fed at. the rate of 1500 cu. ft. per hour and the generator gas at 1300 cu. ft. per hour. There was a definite change in the result produced, in that the austenitic layer which had previously been confined to a position underneath the outer nitride layer was brought to the surface of the steel treated and the nitride layer was absent. It" was, therefore, apparent that with conditions maintained as specified in this test the. result is' an austenitic surface of very advantageous physical properties not previously obtained with certainty and with knowledge of the manner, in which such result had been obtained.

In the next tests inthe series the furnace. tem-r 4 perature was still held at 1300 F., but the N113 increased to 2000 cu. ft. per hour with the generator gas still at 1300 cu. ft. per hour. The results showed that the austenitic layer was still at the outside of the steel displacing the nitride layer entirely but that the austenite, was of a somewhat greater depth. This indicated that there were certain conditions that could be varied Within limits and still be within the range of 0 practical application in which desired results couldbeobtained.

In order more definitely to determine the range over which the austenitic outer layer is formed the, next tests were carried on at 1325" F., with 1000 cu. ft. perhour of NH; and 1300 cu. ft. per

hour of generator. gas. However, the results of these tests indicated that a slight outside nitride layer was present and that therefore this condition was not within the range producing desried results. However, when the NHs was increased to.,1500. cu. ft. per hour with the temperature stillatl325 F. andthe generatorgas 1300 cu. ft. per hour, the resultwas anouter-austeniticlayer similarv to the last two tests at the-1300 temperature. This was also true ofa test withthe 1325 F. temperature, 200.0..cu. ft. per hour of NH3 and 1300 cu. ft. per hour of generator gas.

Later tests were carried out. atv 1.35.0 with 1000 cu. ft. per hour of. generator gas followed by 1500 cu. ft. per hour and,2000cu. it; per hour of N53 with 1350 F. furnace-temperature and 1300 cu. ft. per hour generator. gasremaining the same. The result was; that; the 1500 and 2000 cu. ft. per hour of NH: tests still maintained the outer austeniticlayen The next seriesof tests,- was at 1400? F. with 1000, 1500 and 2000. cu. ft. ofN-Ha, respectively, and a constant 1300 cu. ft. of generator gas. In this series of tests, however, the inner martensitic layer was carried definitely outward to displace the austenitic layer and resulted inan undesirable condition tending toindicate that the range terminatedat 1350? F. with 2000. cu.- ft. per hour of NI-Ia to1300. cu. ft. per hour of generator gas, but still further tests indicate that by increasing the volume of-NHa and generator gases. the temperature may be increased. to 1450 F. or 1500" F. and the desiredresults achieved. The ranges in which the desired outer austenitic layer were obtainedindicates desired results are obtainable with the temperature of 1300' to 1500 F., 1500 to 3000cu. ft. per. hour of'NI-Ia, and 1300 to 1.700 cu. ft. per hourof generator gas.

The analysis of the generator gastused in the above testswas, as follows:

Percent CO2 0.5 02 0.00 CO 15 to 20 CH4 1 to 2 H2 25 Ni -1 55 It is considered, that the factors which must be, controlled to. assure success anddefinite and consistent resultsin the use ofthisprocess'requires satisfactory control ofthetemperature, the ratio of ,NI-Ia to generator gas. and the further fact, that therate of-cooling must be-satisfactorily controlled.

The tests whichwere onduc eddeterminedthe range over which satisfactory results would be obtainedwith the-furnace-used in:. he,=, tests; and. thereiore. theresultsmay besaldto bezempiri al for the furnace used. It is to be understood that variations in the size of the furnace may require variation in the rates of flow of gas. It is also possible that with a greater amount of material treated the amounts might change. It may be stated, however, that for each set of conditions set up by one type of furnace a range can be obtained by a series of tests similar to those outlined herein, and it is therefore definitely established that a range exists which can be ascertained by the methods herein outlined wherein the result will be an austenite layer at the outside of the treated steel. It is very definitely established, however, that satisfactory results are obtained with a temperature range at from 1300 to 1450" to 1500 F. and a ratio of NH3 to converter gas of from 1.1 to 1.8 with a suflicient supply of both gases to assure completion of the desired reactions. Although there are certain advantages in the use of th higher temperature ranges from the standpoint of a reduction in the time cycle there are attendin disadvantages in the possibilities of distortion, particularly of thin sections, in using such higher temperatures. We have therefore preferred to work for production purposes within the lower temperature ranges of from 1300 to 1350 F.

When articles are treated under this process there is a growth in size sometimes amounting to several thousandths of an inch. Taking advantage of this property, it is possible to reclaim some types of worn steel parts and at the same time provide such parts with the exceptional wearing surface which results from the process. Although its use on new parts is of major importance the above mentioned application for reclaiming Worn parts of automotive equipment is of material importance.

It is to be emphasized that although a steel of austenite structure has been previously successfully produced from steels of special composition and heat treatment, the process herein disclosed produces, as far as we are aware, a new product, namely, a case hardened steel of a composition generally used in automotive and other steel practice which has a case of exceptional wearing quality assured by an austenitic surface structure. The portions adjacent the outer austenitic case are generally followed by layers of steel in states produced at the various critical ranges in their regular order, namely, martensite, troostite, sorbite, pearlite and ferrite, the ferrite structure interspersed by nitride needles making up the major portion of the article inside the outer case. The product resulting from the process is a new and important article of manufacture having widespread possibilities of application not only in the automotive, but in all industries where steel of high wearing qualities is desired.

It is of further importance that the combination of increase in size with the formation of a surface structure of superior wearing quality is possible and this makes the process particularly applicable to the reclamation of worn parts.

Articles produced by the above process, in addition to possessing the highly desirable excellent wearing qualities of austenitic steels have also exhibited exceptional and unexpected resistance to corrosion. This is due to the fact that durin the heat treatment the nitrogen penetrates into the steel and form a solid solution which not only lowers the critical transformation temperature, but also results in the production in the final product of a metal of excellent non-corrosive properties.

Therefore, although we have described the invention as applied to a specific installation, it is understood that variations are intended within the scope of the following claims.

We claim:

1. A method of treatment of steel for the purpose of producing a wear and corrosive resistant outer surface which comprises the heating of the steel within a temperature range of 1300 to 1500 F. in an atmosphere of NH; and generator gas in amounts within the range from 1500 cubic feet per hour of NH3 and 1300 cubic feet per hour of generator gas as a lower limit to 3000 cubic feet per hour of NH3 and 1700 cubic feet per hour of generator gas as an upper limit, said atmosphere being substantiall free of hydrocarbons, thereby causing the formation of a wear and corrosive resisting outer surface layer of austenite on such steel.

2. A method of treatment of steel for the purpose of producing a wear-resistant outer surface thereof which comprises the heating of the steel within a temperature range of 1300 to 1500 F. in an atmosphere of NH3 and generator gas in ratios within the range from 1.1 parts NHs to 1.0 part generator gas as a lower limit to 1.8 parts NHa to 1.0 part generator gas as an upper limit said atmosphere being substantially free of hydrocarbons and employed in amounts sufficient to cause the formation of a wear and corrosive resisting outer surface layer of austenite on such steel.

GOSTA VENNERHOLM.

HARRY D. FORD. FREDERICK C. YOUNG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,745,104 Machlet Jan. 28, 1930 1,995,314 Machlet Mar. 26, 1935 2,102,539 Lauenstein et al. Dec. 14, 1937 2,114,802 Kinzel Apr. 19, 1938 2,151,190 Cowan Mar. 21, 1939 2,188,226 Machlet Jan. 23, 1940 FOREIGN PATENTS Number Country Date 419,057 Great Britain Oct. 31, 1934 443,524 Great Britain Mar. 2, 1936 OTHER REFERENCES Nitrocementation of Steel, by Braun, Vlasov and Goldine, published in Metallurgy No. 7, 1940, and discussed in Metallurgie, Feb. 1941, pages 99-102.

Transactions of American Society for Steel Treating Nitriding Symposium, Oct. 1929, pages 1-17.

Transactions American Society for Metals, Cleveland, Ohio, vol. 26, 1938, pages 767 and 771. 

